Our approach to understanding drug addiction has targeted the dopamine and opioid systems as among the major transmitters having a role in the reinforcing effects of drugs of abuse. The actions of these transmitters are mediated by receptors that belong to the G protein coupled receptor (GPCR) family and form direct or indirect targets of drug abuse. Thus, since the receptors are central to the homeostasis, regulation and signaling by these transmitters, our work has focused on the elucidation of the function of dopamine and opioid receptors. Our studies of these receptor systems revealed that they exist and function within homoooligomeric and heterooligomeric complexes. We have established that there is specificity of both homo- and hetero-oligomerization allowing certain receptors to either interact or stay separate. We are now involved in determining the rules governing the specificity underlying receptor-receptor interactions. Thus far, we have determined that transmembrane domain four is a critical part of the dimer interface but additional structural domains also participate in oligomer formation. We will investigate this using a variety of techniques, including crosslinking, immunoprecipitation, fluorescence resonance energy transfer, laser confocal microscopy and a novel methodology we have developed. We will determine the forms of the receptors (monomers, dimers, oligomers) that exist at the cell surface. In addition, we will determine whether the receptors are contained in membrane microdomains such as lipid rafts. We have shown that receptor heterooligomerization can result in entirely novel signaling and pharmacological properties distinct from that of either receptor partner. We aim to determine the physiological role of these heterooligomerization complexes in native brain tissue, neuronal cultures and transfected cells. We are specifically interested in investigating and comparing the receptor structural determinants underlying both homo and heterooligomerization. Oligomerization represents a highly significant complexity in receptor signal transduction mechanisms that is critical to study, not only to further the understanding of physiological processes, but to eventually incorporate these aspects into the search for new therapeutic agents.